1. Field of the Invention
The present invention relates to the field of mode-switching transformers which are used to convert a voltage from a common mode to a differential mode and conversely.
2. Discussion of the Related Art
FIG. 1 is a schematical block diagram of a transformer of the type to which the present invention applies.
It is a structure for converting a common mode voltage Vc referenced to earth M or to ground into a differential voltage Vd provided between two output terminals 2, 3. A common mode input-output terminal 4 is, for example, floating (in an application to radiofrequency signal reception).
The present invention more specifically relates to such so-called distributed or BALUN (balanced-unbalanced) converters formed from coupled conductive tracks. Such converters are of so-called Marchand type and they are known to be wide-band converters.
FIG. 2 shows the electric diagram of a Marchand-type mode-switching converter to which the present invention applies. On the common mode side (inputs 4 and earth M), a first winding formed of two sections 5 and 6 of conducive spirals is coupled with a second winding also formed of two sections 7 and 8 on the differential mode side. Sections 7 and 8 are electrically in series between terminals 2 and 3 and exhibit a junction point 9 forming the ground of the differential mode (possibly isolated from the common mode earth). The lengths of the different windings are all identical, which characterizes a Marchand-type converter. These lengths are all equal to λ/4, where λ corresponds to the wavelength of the central frequency of the passband of the considered converter.
The relation linking wavelength λ to frequency is:λ=c/f√{square root over (∈)}, where c represents the velocity of light in vacuum, where ∈ represents the substrate permittivity and where f represents the frequency (here, the central frequency of the desired passband).
In a converter of the type in FIG. 2, this central frequency is defined as being the median frequency of the passband corresponding to ±3 dB with respect to the minimum attenuation of the transformer.
An example of a Marchand-type mode-switching transformer to which the present invention applies is described, for example, in artide “A Silicon Monolithic Transmission Balun with Symmetrical Design” by Yeong J. and Yichenglu, published in IEEE Electron Devices Letters, volume 20, N°4, in April 1999.
A disadvantage of mode-switching transformers formed of conductive tracks on different metallization levels in integrated structures is linked to the size taken up by these transformers. For example, for frequencies on the order of one gigahertz, it is frequent to have to size the lines with lengths of several centimeters, which is far from being negligible in the bulk of an integrated or printed circuit.
Another difficulty is the fact that the longer the sections, the higher the insertion losses. Now, in integrated applications (for example, mobile telephony), the received signals are often of small intensity and insertion losses thus must be minimized.